I updated my dom0 last night, and shortly after the domUs started up, it
triggered another KASSERT. It's been running OK since a reboot, though, so
it looks like it's not quite as easy to trigger as the earlier ones (and may
or may not be a new problem).

I had tried running a -current dom0 kernel and hit this panic twice
fairly quickly. It appears related to disk I/O, and the 2 VMs I was
running are fairly heavy on disk I/O. I switched back to a 5.1_STABLE
dom0 for a while, but I looked at that KASSERT a bit, but wasn't able to
follow all the code all that well. The KASSERT had been added somewhat
recently. I changed the KASSERT to an #ifdef DIAGNOSTIC check so I at
least wouldn't panic, and while testing the MP domu kernel had a couple of
hits with this check last night: